The present invention relates generally to video cassette recorders (VCR's) and particularly concerns the operation of such circuits in the presence of encoded data such as teletext and closed-caption data.
The phenomenon of drop-out during VCR playback is well known in the art. In conventional consumer VCR's, video signals are recorded on a magnetic tape and later played back in the form of frequency modulated RF signals. Drop-out occurs when the level of the RF signal falls below a given threshold value. This can be caused by various factors including imperfections in the tape (ie. when the magnetic particles "drop-out" from the tape), dust adhering to the tape and improper tension of the tape during playback.
The use of special circuits to compensate for such drop-outs is well known in the art. In particular, prior art drop-out compensation circuits typically operate in reliance on the correlated nature of video signals by substituting a video patch for the drop-out, the patch being derived from a portion of the video signal nearby the drop-out. For example, the patch may be derived from a prior segment of the same video line on which the drop-out occurred or may be derived by averaging the video comprising the line on which the drop-out occurred with the video occurring on the previous line. In any case, drop-out compensation is effected on the assumption that the typical video signal is highly correlated in a spatial sense.
This assumption, however, is not valid for all components of a video signal and, in particular, is not valid in the case of data encoded in the video signal. Such data, for example closed-caption data encoded on line 21 and teletext data encoded on lines 17-19 of the vertical blanking interval (VBI) of an NTSC television signal, is not spatially correlated so that the use of a conventional drop-out compensation circuit in connection therewith will corrupt the data beyond the point of recognition. A similar problem occurs in noise cancellation circuits used in many VCR's, since such circuits also operate on the presumption of correlated video.
It is therefore a basic object of the present invention to provide an improved VCR play-back circuit.
It is a more specific object of the invention to provide an improved VCR which operates in a manner that minimizes the corruption of encoded data during playback.